From: Eric Dumazet <eduma...@google.com>
Date: Tue, 12 May 2020 06:54:30 -0700
> We autotune rcvbuf whenever SO_RCVLOWAT is set to account for 100%
> overhead in tcp_set_rcvlowat()
>
> This works well when skb->len/skb->truesize ratio is bigger than 0.5
>
> But if we receive packets with small MSS, we can end up in a situation
> where not enough bytes are available in the receive queue to satisfy
> RCVLOWAT setting.
> As our sk_rcvbuf limit is hit, we send zero windows in ACK packets,
> preventing remote peer from sending more data.
>
> Even autotuning does not help, because it only triggers at the time
> user process drains the queue. If no EPOLLIN is generated, this
> can not happen.
>
> Note poll() has a similar issue, after commit
> c7004482e8dc ("tcp: Respect SO_RCVLOWAT in tcp_poll().")
>
> Fixes: 03f45c883c6f ("tcp: avoid extra wakeups for SO_RCVLOWAT users")
> Signed-off-by: Eric Dumazet <eduma...@google.com>
Applied and queued up for -stable, thanks Eric.
